Method and appartaus for controlling driving power of motor vehicle

ABSTRACT

A driving power control apparatus for a motor vehicle. For meeting acceleration performance demanded by a driver of the motor vehicle, engine torque produced by an internal combustion engine and driving torque of the motor vehicle determined by the engine torque and a speed change ratio of a transmission are controlled coordinately for thereby allowing acceleration satisfying exactly the driver&#39;s demand.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method of controlling a driving powerof a motor vehicle and an apparatus for carrying out the method.

In general, the driving power of a motor vehicle can be controlledthrough combination of a transmission with an internal combustionengine.

In the case of the driving power control system known heretofore,however, a serious problem has arisen with regard to operationperformance or maneuverability of the motor vehicle because the engineand the transmission are controlled separately independent of eachother.

Under the circumstances, there has recently been proposed an attempt forimproving the maneuverability of the motor vehicle by increasing ordecreasing the output power or torque of the engine upon changing of thevehicle speed by shifting up or down correspondingly through the speedranges of the transmission, as is disclosed in JP-A-64-4544.

However, the proposed system still suffers from a disadvantage that theactual operation of the motor vehicle can not satisfactorily follow themaneuverability demanded by the driver to such extent that he or shefeels comfortable in the way the vehicle drive.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a methodof controlling driving power of a motor vehicle such that a driver canexperience a feeling of congruence between the maneuverability of amotor vehicle demanded by the driver and the actual operation of thevehicle sufficient to realize enhanced comfortableness of driving. It isanother object of the invention to provide an apparatus for carrying outthe method.

In view of the above and other objects which will be apparent as thisdescription proceeds, there is proposed according to an aspect of theinvention an apparatus for controlling the driving power of a motorvehicle, which apparatus comprises engine torque control means forcontrolling engine torque outputted from an internal combustion engineof the motor vehicle, driving torque control means for controlling adriving torque for driving the motor vehicle on the basis ofcombinations of the engine torques and transmission gear ratios of atransmission, and torque control means for causing at least one of theengine torque control means and the driving torque control means toperform torque control in response to a desired torque demanded by adriver on the basis of the engine torque or the driving torque or thecombination of the engine torque and the driving torque.

With the structure of the driving power control apparatus for the motorvehicle described above, a feeling of comfort in driving the motorvehicle can be enhanced because of the capability of controlling theengine torque and/or driving torque so as to follow the torquecharacteristics demanded by the driver.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing schematically the general arrangement of anapparatus for controlling a driving power of a motor vehicle accordingto an embodiment of the invention together with a power transmissiontrain of the vehicle;

FIG. 2 is a view for graphically illustrating the basic conceptunderlying a driving torque control apparatus according to an embodimentof the invention;

FIGS. 3 to 5 are flow charts for illustrating a control procedure to beexecuted on the basis of the control scheme shown in FIG. 2;

FIG. 6 is a view showing graphically relations existing between drivingtorques and accelerator pedal strokes with speed ranges changed over bya transmission being used as a parameter;

FIG. 7 is a view showing a map of throttle opening degrees incorrelation with engine rotation numbers and engine torques;

FIG. 8 is a view for graphically illustrating fuel-cost characteristicsas a function of engine rotation or revolution number and engine torque;

FIG. 9 is a view for graphically illustrating the principle of a controlapparatus according to the invention for a motor vehicle in which astepless transmission (torque converter) is employed;

FIG. 10 is a flow chart for illustrating a procedure for realizing thecontrol scheme shown in FIG. 9;

FIG. 11A is a characteristic diagram for graphically illustrating aconventional driving torque control scheme;

FIG. 11B is a characteristic diagram for graphically illustratinganother driving torque control scheme according to the invention incomparison with a corresponding control known heretofore and shown inFIG. 11A;

FIG. 12 is a flow chart for illustrating a control procedure forrealizing the control scheme shown in FIG. 11B;

FIG. 13 is a characteristic diagram for illustrating relations existingamong vehicle speeds, driving torques and speed ranges of atransmission;

FIG. 14 is a view for illustrating graphically another example of acontrol scheme according to the invention;

FIG. 15 is a flow chart for illustrating a procedure for executing thecontrol shown in FIG. 14;

FIG. 16 is a flow chart for illustrating a control procedure to beexecuted additionally upon occurrence of an abnormality in the course ofexecution of the control procedure shown in FIG. 12;

FIG. 17 is a functional block diagram showing a control system accordingto yet another embodiment of the invention;

FIG. 18 is a flow chart for illustrating a control procedure performedby the control system shown in FIG. 17;

FIG. 19 is a view for graphically illustrating the concept of a furtherexample of the control scheme according to the invention;

FIG. 20 is a functional block diagram showing a control system accordingto a still further embodiment of the invention;

FIG. 21 is a flow chart for illustrating a control procedure executed bythe system shown in FIG. 20;

FIG. 22 is a view for graphically illustrating the concept of anothercontrol scheme according to the invention;

FIG. 23 is a functional block diagram of a control system designed forrealizing the control scheme shown in FIG. 22;

FIG. 24 is a flow chart for illustrating a control operation performedby the system shown in FIG. 23;

FIG. 25 is a view showing graphically still another control schemeaccording to the invention; and

FIG. 26 s a flow chart for illustrating the control procedure forexecuting the control scheme shown in FIG. 25;

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the present invention will be described in detail in conjunctionwith exemplary or preferred embodiments thereof by reference to theaccompanying drawings.

FIG. 1 is a schematic diagram showing a driving torque control apparatusaccording to an embodiment of the invention together with a powertransmission train thereof. Referring to the figure, a signal indicatingmagnitude of depression or actuation, i.e. a depression angle, of anaccelerator pedal 1 (hereinafter also referred to as the acceleratorpedal stroke) is inputted to a driving torque control unit 2 and/or anengine torque control unit 3. Controlled by the engine torque controlunit 3 are a throttle control unit 6 (constituted by an electric motor,a throttle valve and a throttle sensor) which is installed in an intakepipe 5 leading to an internal combustion engine (hereinafter referred toas the engine) 4, a fuel injection system 7A and an ignition system 7B,whereby the engine output torque is controlled. On the other hand, thedriving torque control unit 2 is adapted to operate or drive through aproportional or duty control an automatic transmission 30, a shiftsolenoid 8, a lock-up solenoid 9 and others, and additionally serve forconnection and disconnection of a speed changing clutch 10 and a lock-upclutch 11, whereby a speed change control is performed. Throughcombination of the speed change control with the engine torque controlin this manner, an intended driving torque control is realized.

The driving torque control based on the combination of the speed changecontrol and the engine torque control (preferably the engine torquecontrol based on the control of a degree of opening of the throttlevalve) is performed on the basis of the speed of the motor vehicleindicated by a signal produced by a rotation sensor 12 installed inassociation with a drive shaft and the accelerator pedal stroke (i.e.magnitude of depression of the pedal). In the case of the illustrativeembodiment, the engine torque control unit 3 and the driving torquecontrol unit 2 are electrically interconnected for mutual communicationthrough a local area network (LAN). Of course, when a high-performancecomputer such as 32-bit computer is employed, desired coordinatedcontrols of the engine and the transmission train may be performed by asingle integral control unit, as indicated generally by a block 13 inFIG. 1.

FIG. 2 is a view for graphically illustrating the basic conceptunderlying the driving torque control according to an embodiment of theinvention. In the figure, the speed of the motor vehicle is taken alongthe abscissa while the accelerator pedal stroke and hence the drivingtorque are taken along the ordinate. In FIG. 2, solid lines representthe driving torques in the state in which the throttle valve is fullyopened, wherein an interval [0-a] represents a first speed range, aninterval [a-b] represents a second speed range, an interval [b-c]represents a third speed range and a following interval [c→] representsa fourth speed range on the assumption that the torques to be controlledby the actuation of the accelerator pedal are classified into thedriving torques in the vehicle speed intervals [0-a] and [a-b],respectively, and the engine torques in the vehicle speed intervals[b-c] and [c→], respectively, as shown along the ordinate. Accordingly,when a driver has depressed the accelerator pedal to a value B (pedalstroke B) in the speed interval [0-a], the transmission gear ratio isset to the third speed range with the throttle valve being fully opened,because the desired driving torque corresponding to the acceleratorpedal stroke is the torque B. On the other hand, when the vehicle speedis higher than b, there is no longer available a driving torque havingvalues C to D even when the accelerator pedal is depressed to a maximum.Accordingly, the desired or demand torque corresponding to thedepression of the accelerator pedal will have to be provided as anengine torque. So far as the vehicle speed lies within the interval[0-b], it is required to perform a control on the basis of a combinationof the driving torque and the engine torque.

FIGS. 3 to 5 are flow charts for illustrating execution of the controlscheme described above. Referring to the figures, the accelerator pedalstroke α which indicates the torque demanded by the driver is fetched ata step S1, being followed by a step S2 where the engine rotation numberN_(e) is detected and a step S3 where the vehicle speed V is detected,respectively. At a step S4, it is decided whether the vehicle speed V isequal to or lower than the speed a shown in FIG. 2. When the result ofthis decision is affirmative (Yes), i.e. when V is equal to a or fallswithin the interval [0-a], a decision is then made at a step S5 whetherthe accelerator pedal stroke α is smaller than the value A (FIG. 2),inclusive thereof. If so (Yes), a signal is outputted which commandsthat the transmission gear ratio (converter drive position) be set tothe fourth speed range. The desired or target value of the drivingtorque corresponding to the accelerator pedal stroke α at that time canbe determined by consulating a driving torque vis-a -vis acceleratorpedal stroke map shown in FIG. 6. At a step S7, the engine torque T_(e)is determined on the basis of the accelerator pedal stroke α inaccordance with a function f (α) prepared by taking into account areduction gear ratio and other factors. At a step S8, the opening degreeθ_(th) of the throttle valve is determined on the basis of the enginetorque T_(e) thus determined and the engine rotation number N_(e) byconsulting a map of throttle opening degrees (θ_(th)) shown in FIG. 7,whereon a signal indicating the throttle opening degree thus determinedis outputted to the throttle control unit 6 at a step S9 to therebyallow the driving torque demanded by the driver to be outputted.

On the other hand, when it is determined at the step S5 that theaccelerator pedal stroke α is greater than A, a decision is then made ata step S10 as to whether or not the accelerator pedal stroke α is equalto or smaller than the value B (FIG. 2). If so, the third speed rangecommand is issued at a step S11, being followed by a step S12 where theengine torque T_(e) is determined through similar procedure as that ofthe step S7 described above, whereon the control processing proceeds tothe step S8.

When the decision at the step S10 results in that the accelerator pedalstoke α is greater than B, it is then decided at a step S13 whether α isequal to or smaller than C. If so, a second speed range command isoutputted at a step S14, which is then followed by a step S15 at whichthe engine torque T_(e) is determined similarly to the step S7, whereonthe control processing proceeds to the step S8.

On the other hand, when the decision at the step S13 shows that theaccelerator pedal stroke α is greater than C, a first speed rangecommand is issued at a step S16, which is then followed by a step S17for determining the engine torque T_(e), whereon the processing proceedsto the step S8, as described above.

In the foregoing, it has been assumed that the accelerator pedal strokeα is changed in the state where the vehicle speed is within the interval[0-a].

In contrast, when it is determined at the abovementioned step S4 thatthe vehicle speed is higher than a, the control procedure branches to aprocessing flow shown in FIG. 4, as indicated by L.

Now, referring to FIG. 4, decision is made at a step S18 as to whetherthe vehicle speed V is equal to or lower than b.

When it is determined from the decisions at the steps S4 and S18 thatthe vehicle speed falls within the interval [a-b], decisions are thenmade as to whether the accelerator pedal stroke α exceeds the values Aand B at steps S19 and S22, respectively.

Subsequently, at succeeding steps S20; S21 to S25; S26, processingssimilar to those at the steps S6 to S17 are executed, being followed bythe steps S8 and S9, as described above by reference to FIG. 3.

At this juncture, it is to be noted that the first speed range is notselected within the vehicle speed interval [a-b].

When decision is made at the step S18 in FIG. 4 that the vehicle speed Vis higher than b, the control procedure then branches to a processingflow illustrated in FIG. 5, as indicated by M, whereon it is decided ata step S27 whether the vehicle speed is lower than c inclusive.

When it is determined that the vehicle speed falls within the interval[b-c], then decision is made at a step S28 whether the accelerator pedalstroke α is smaller than A inclusive, which is then followed byprocessing steps S29 to S32 as well as the steps S8 and S9, whereby thesimilar processing described hereinbefore in conjunction with FIG. 3 isexecuted.

In this vehicle speed interval [b-c], neither the first nor the secondspeed range is selected.

Finally, when it is determined at a step S27 that the vehicle speed ishigher than c, the processing jumps directly to the step S29 at whichthe fourth speed command is issued, whereon the throttle opening degreeθ_(th) is determined.

FIG. 8 is a view for graphically illustrating isometrical fuel-costcharacteristics as a function of the engine rotation numbers andtorques. In this figure, the engine rotation number per unit time(engine speed) is taken along the abscissa with the engine torque beingtaken along the ordinate. In the figure, a broken line curve representsthe engine torque produced when the throttle valve is fully opened.Further, in this figure, a hatched area represents a region in which thefuel-cost ratio is most preferable. Besides, a solid line curve krepresents a path of transition to the hatched area along which the mostpreferable fuel-cost ratio can be assured. As can be seen from thiscurve k, in order to realize the most preferable fuel-costcharacteristic, it is required to increase the engine torque up to apoint corresponding to about two thirds of the maximum torque of thehatched area while maintaining the engine rotation number at 1000 rpm,wherein upon the engine torque reaching the abovementioned point, theengine rotation number and the engine torque are both increasedcoordinately to the respective maximum values corresponding to thehatched area. By controlling the driving torque in this manner,economization of fuel can be improved significantly because of theoperation which can follow the most preferable fuel-cost characteristiccurve k.

FIG. 9 is a view for graphically illustrating a relation between thevehicle speed and the driving torque in a motor vehicle in which astepless transmission (also known as torque converter or CVT for short)is employed. As can be seen from the figure, the transmission gear ratiocan be changed freely in the case of the stepless transmission.Accordingly, there is no appearance of a stepwise change in the drivingtorque (i.e. the latter can be varied continuously or smoothly).

The maximum value of the driving torque which changes as a function ofthe vehicle speed is set to a value to correspond to the maximumaccelerator pedal stroke. A curve shown in FIG. 9 represents the targetor demanded driving torque at given vehicle speeds.

More specifically, when the motor vehicle is running at a speed a, thedriving torque D represents the maximum value of the driving torquedemanded by the driver who has pushed down or floored the acceleratorpedal to a maximum. The angle of the accelerator pedal relative to thefloor is represented by C at that time. When the transmission gear ratiois changed over from H₁ to H₂ in this state, the driving torque can bechanged in the state where the throttle valve is opened fully.

When the accelerator pedal stroke is A at the vehicle speed b, thetransmission gear ratio is set to H₄ with the throttle valve beingopened fully.

FIG. 10 is a flow chart for illustrating a control procedure for themotor vehicle in which the stepless transmission or torque converter isemployed. Referring to the figure, the vehicle speed V_(SP) and theaccelerator pedal stroke (α) are detected at steps S33 and S34,respectively. Therefore, a map of the relations between the vehiclespeeds and the accelerator pedal strokes (desired driving torques) suchas shown in FIG. 9 is retrieved at a step S38. Subsequently, at a step36, it is decided whether the driving torque indicated by the detectedaccelerator pedal stroke α is equal to or greater than the drivingtorque at the upper limit transmission gear ratio H₄ (i.e. the demandeddriving torque A). If so, the throttle opening degree θ_(th) is set to amaximum value at a step S37, whereon the transmission gear ratiocompatible with the vehicle speed and the accelerator pedal stroke isselected at a step S38 that α<A, and the throttle opening degree θ_(th)is determined on the basis of the engine torque T_(e) =f(α). Thetransmission gear ratio is set to the upper limit H₄.

In this manner, according to the teaching of the invention as employedin the embodiment described above, there can be realized the feeling ofcongruence between the maneuverability demanded by the driver and theactual operation performance of the motor vehicle.

Next, description will be made of a modified embodiment of theinvention.

FIGS. 11A and 11B are views for graphically illustrating relationsbetween the driving torque and the throttle opening degree in torquecontrol systems according to the prior art and the invention,respectively. Referring to the figures, it will be seen that therelation between the throttle opening degree and the driving torquebecomes different in dependence on the speed ranges, as indicated bybroken line curves. According to the control scheme known heretofore, asthe driving torque is progressively increased by pushing down graduallythe accelerator pedal, the speed ranges of the transmission are shifteddown from one to another. In contrast, the control system according tothe instant embodiment of the invention is designed to operate such thateven when the driving torque is increased, the current speed range ofthe transmission is maintained until the throttle valve is fully opened,whereon the speed range is shifted down only when a higher drivingtorque is demanded, as is shown in FIG. 11B. At this time, however, thethrottle valve is once closed and subsequently opened again when thedriving torque is further increased, which is then followed by theshift-down of the speed range. This process is repeated. With thiscontrol scheme, the driving torque is protected against changing rapidlyor abruptly. This control scheme is very advantageous from thestandpoint of the fuel cost ratio because the fully opened throttlerange is made use of predominantly in combination with the high speedrange, whereby the engine rotation number can be maintained low, asdescribed hereinbefore in conjunction with FIG. 8. Further, because theopening degree of the throttle valve is so controlled upon speed changethat the torque is prevented from changing rapidly or abruptly, shockdue to the speed change can be mitigated.

FIG. 12 is a flow chart for illustrating operation of a control systembased on the control concept illustrated in FIG. 11B. Referring to FIG.12, in response to the input of a timer interrupt signal, theaccelerator speed and the vehicle speed are first measured at a stepS50. Subsequently, at a step S51, the driving torque and the speed rangefor the transmission are calculated on the basis of relations shown inFIG. 13 which shows graphically the relations existing among the vehiclespeeds, the driving torques and the speed ranges of the transmission,wherein the maximum driving torques at given vehicle speeds areindicated by solid line curves. The driving torque is calculated on thebasis of the maximum driving torque corresponding to the current vehiclespeed and the accelerator pedal stroke to thereby determine the speedrange of the transmission. At this juncture, it should be mentioned thatbecause the throttle valve is actuated frequently to the fully orapproximately fully opened state, the speed ranges of the transmissionmay be changed over many times due to noise accompanying the voltagesignal indicating the accelerator pedal stroke, giving rise tooccurrence of a chattering phenomenon which must of course be prevented.To this end, the speed range of the transmission may be determined forthe shift-up on the basis of the broken line curves shown in FIG. 13 atsteps S52 and S53. By providing previously such hysteresis asillustrated in FIG. 13, the chattering phenomenon which would otherwiseoccur upon speed changing can positively be suppressed, whereby thetransmission can be protected against abrasion or wear. Next, at a stepS54, the engine torque is arithmetically determined on the basis of aproduct of the driving torque and the transmission gear ratio.Subsequently, at a step S56, a map prepared previously at a step S55 forthe relations of the throttle opening degrees to the engine rotationnumbers and the engine torques is consulated to determine the throttleopening degree, as a result of which signals indicative of the throttleopening degree and the speed range are outputted for controlling theengine output torque and the transmission gear ratio, respectively.

FIG. 14 is a view similar to FIG. 11B and illustrates graphically theconcept of another control scheme for controlling the speed change andthe throttle valve operation. According to this control scheme, thetransmission (speed range) and the throttle opening degree arecontrolled on the basis of the accelerator pedal stroke without takinginto consideration the vehicle speed.

FIG. 15 shows a flow chart for illustrating the control scheme. At astep S50, the accelerator pedal stroke and the engine rotation numberare measured, which is then followed by a step S57 where it is decidedwhether or not the maximum engine rotation number Maxrpm is exceeded.When the result of the step S57 is affirmative (Yes), the speed range isshifted up by one step for protecting the engine against abrasion at astep S59. If otherwise (No), the speed range and the throttle openingdegree are determined on the basis of the relations illustrated in FIG.14. Subsequently, at a step S61, the signals indicating the speed rangeand the throttle opening degree thus determined are outputted forcontrolling the transmission gear ratio and the engine output torque.This control scheme is much simplified as compared with the scheme shownin FIG. 12 and is advantageous over the latter in that the time takenfor the arithmetic determination can be significantly shortened.

FIG. 16 is a flow chart for illustrating a procedure of processing to beexecuted in case the engine rotation number becomes excessively largedue to abnormality of operation of the throttle valve and thetransmission occurring in the course of the control performed inaccordance with the scheme shown in FIG. 12. To this end, a timerinterrupt is issued for measuring the current transmission gear ratioand engine rotation number at a step S62. When it is decided at a stepS63 that the engine rotation number exceeds a maximum engine rotationnumber Maxrpm, the speed range is shifted up through processing stepsS64 and S65 to decrease the engine rotation number for therebyprotecting the engine from wear or abrasion. At that time, theabnormality may be messaged to the driver by lighting an alarm lamp.

Now, description will be turned to exemplary applications of theteachings of the present invention.

FIG. 17 is a functional block diagram showing a deceleration controlsystem. A reference numeral 33 denotes a deceleration decision functionfor making decision whether or not a driver desires deceleration of amotor vehicle on the basis of outputs from an accelerator pedal strokedetecting block 31 and a braking force detection block 32. Subsequently,a level of deceleration demanded by the driver is determined by adeceleration determining block 34. In case the level of decelerationdemanded by the driver corresponds to a minimum or lowest speed, asignal indicating a speed range which can assure the highest fuel costefficiency such as a neutral position N is outputted to a speed changecontroller 35. When the deceleration demanded by the driver exceeds acertain level, a speed change ratio arithmetic unit 36 outputs a speedchanging ratio signal corresponding to the demanded deceleration to thespeed change controller 35. Further, such a throttle opening degreewhich can prevent occurrence of shock upon speed change is determined bya throttle opening degree arithmetic unit 37 to be outputted to thethrottle valve controller 36.

FIG. 18 shows a flow chart for illustrating the control processingperformed by the control system shown in FIG. 17. Referring to FIG. 18,there are first fetched an accelerator pedal stroke α, a braking forceB, a vehicle speed V and an engine rotation number N_(e) at a step S70.Subsequently, decision is made at a step S71 as to whether the vehiclespeed V is equal to or higher than a predetermined speed V₀. When V≧V₀,then it is decided at a step S72 whether the accelerator pedal stroke αis zero. In case the accelerator pedal stroke α is zero, indicating thatthe accelerator pedal is not pushed down or actuated, a flag "Flg" isset at steps S73 and S75. During a given period of X msec determined ata step S74, the current speed change ratio i_(now) is outputted at astep S76. Subsequently, after lapse of the duration of X msec, asdetermined at the step S74, the flag "Flg" is cleared at a step S77,whereon the value of the braking force β is detected at the step S70.When it is found at a step S78 that the braking force β is smaller thana predetermined value β₀, it is determined that the driver demands theinertia deceleration (inertia brake), whereby the transmission is set tothe neutral position (i_(m) =N) at a step S79. Subsequently, whendecision is made at a step S80 that the braking force β is equal to orsmaller than β₁, the transmission gear ratio i₁ is determined inaccordance with a function f(V, N_(e)) of the vehicle speed V and theengine rotation number N_(e) at a step S81. Next, at a step S82, thethrottle opening degree θ_(th) is determined in accordance with afunction g(V, i_(l)) of the vehicle speed V and the transmission gearratio i_(l) to output the throttle opening degree θ_(th) as determined,being followed by a step S83 for outputting the transmission gear ratioi_(l). Thereafter, the transmission gear ratio and the throttle openingdegree are determined, every time the braking force is changed, throughthe control procedure described above. In case the braking force β isfound to be a maximum β_(m) at a step S84, indicating that the driverdemands the maximum braking force, then the transmission gear ratio andthe throttle opening degree are so determined that the engine rotationnumber is decreased through the processing steps S85, S86 and S87. Thevalues β₀ to β_(m) are set to values which are proportional to thetransmission gear ratios of the automatic transmission (torqueconverter) which are capable of being controlled.

FIG. 19 is a view for graphically illustrating the concept of anoverload reduction control system to which the present invention can beapplied. In a stepwise automatic transmission in which a torqueconverter is used, there exists an overload area in the low speed stateof the first speed range, as indicated by a hatched area. Consequently,it is required to decrease the maximum demand value of the drivingtorque (the upper limit of the opening degree of the throttle valve) inorder to prevent occurrence of impulsive torque at the start of themotor vehicle. By adopting this control scheme, the second, third andfourth speed ranges can be used more frequently at a lower range of thedesired driving torque (at small accelerator pedal strokes) whilemitigating fatigue which the driving system may experience.

FIG. 20 is a functional block diagram showing the overload reductioncontrol system. Referring to the figure, the decelerator pedal stroke isdetected by an accelerator pedal stroke detecting part 31, whereondecision is made by an overload decision function block 40 as to whetheror not the overload control should be performed. When the overloadcontrol is necessary, the throttle opening degree is determined by thethrottle opening degree arithmetic function block 37, which is thenfollowed by determination of a corrected throttle opening degree by adriving torque correcting part 42. The corrected throttle opening degreethus determined is outputted to the throttle valve controller. Thedriving torque correcting part 42 is supplied with a torque ratio of thetorque converter which is determined by a torque ratio arithmetic part41, whereby the torque ratio is corrected correspondingly.

FIG. 21 is a flow chart for illustrating a control procedure based onthe concept shown in FIG. 20. Referring to FIG. 21, the acceleratorpedal stroke α and the engine rotation number N_(e) are fetched at astep S90. When it is found at a step S91 that the accelerator pedalstroke α is equal to or greater than α₁, i.e. when the vehicle speed islower than V₀ (=30 km/h, for example) and that the speed change positionof the transmission falls within the first speed range, the transmissiongear ratio i_(l) is outputted at a step S92, whereon the desired drivingtorque value is determined in accordance with the function f(α) of theaccelerator pedal stroke α at a step S93. Thereafter, the throttleopening degree θ_(th) is determined in accordance with a functiong(t_(t), i_(l), N_(e)) at a step S94. The throttle opening degree θ_(th)thus determined is outputted at a step S95. At that time, the maximumvalue of the driving torque is set at a value which corresponds to themaximum value of the accelerator pedal stroke.

FIG. 22 is a view for graphically illustrating the concept of a drivingtorque correcting control system in which change in the atmosphericpressure is taken into account. Because of difference in the atmosphericpressure between a high land and a low land, the maximum driving torqueundergoes a corresponding change. Under the circumstances, atransmission gear ratio B' at a vehicle speed a is effectuated at avehicle speed b when the vehicle is running on a high land or hill.Thus, a substantially same driving torque is obtained even during therunning on a low land, whereby the driving torque demanded by the drivercan be satisfied notwithstanding of changes in the atmospheric pressure.

FIG. 23 is a functional block diagram of a control system designed forcontrolling the driving torque while taking into consideration thechanges in the atmospheric pressure. The atmospheric pressure isdetected by an atmospheric pressure detection block 43 and inputted to acorrected driving torque arithmetic function block 45 which determines acorrected driving torque on the basis of the atmospheric pressure aswell as the vehicle speed and the engine rotation number detected by thevehicle speed detector 44 and an engine rotation number detector 46,respectively. Subsequently, a corrected transmission gear ratio isdetermined by a corrected transmission ratio arithmetic unit 47 while acorrected throttle opening degree is determined by a corrected engineoutput arithmetic unit 45, whereon the corrected gear ratio and thecorrected engine torque are outputted to a speed change controller 35and a throttle controller 38, respectively.

FIG. 24 is a flow chart for illustrating the control operation performedby the system shown in FIG. 23. Referring to FIG. 24, the atmosphericpressure P, the vehicle speed V and the engine rotation speed N_(e) arefetched at steps S100, S101 and S102, respectively. At a step S103, thecorrected driving torque T₀ is determined in accordance with a functionf(P, V), while the corrected transmission ratio Δ_(i) is determined inaccordance with a function g(T₀, N_(e)) At a step S105, it is decidedwhether h(Δ_(i), θ) determined on the basis of the correctedtransmission ratio Δ_(i) and the throttle opening degree θ_(th)coincides with ΔT₀. When coincidence is found, the transmission ratioΔT_(i) is outputted straightforwardly because correction of the throttleopening degree is unnecessary. On the other hand, when discrepancy isfound at a step S105, the corrected throttle opening degree θ_(th) isdetermined in accordance with a function j(Δ_(i), N_(e)) at a step S106,whereon the transmission ratio Δ _(i) at that time is outputted at astep S108.

FIG. 25 is a view showing graphically the concept of an air-fuel ratiocontrol according to another modified embodiment of the invention. Ascan be seen in the figure, the air-fuel ratio (A/F) is decreased onlywithin a power zone indicated by a hatched area to thereby enrich theair fuel mixture. In the other region, the air-fuel ratio is increasedto make the air-fuel mixture lean.

FIG. 26 is a flow chart for illustrating the air-fuel controlprocessing. Referring to the figure, the transmission gear ratio i, thevehicle speed V and the throttle opening degree θ_(th) are fetched atsteps S110, S111 and S112, respectively. At a step S113, decision ismade as to whether the vehicle speed i falls within the first speedrange i_(l). At a speed S114, it is decided whether the vehicle speed Vis equal to or smaller than a predetermined speed V₀ (e.g. 60 km/h). Ata step S115, it is decided whether the throttle opening degree θ_(th) isequal to or greater than a predetermined value θ_(k). Provided thati=i_(l), V≦V₀ and that θ_(th) ≧θ_(k), richness (A/F=12) is determinedfrom an air-fuel ratio map at a step S116, whereon a corresponding fuelamount signal T_(p) is outputted at a step S117. Otherwise, leanness(A/F=19) is determined from the air-fuel ratio map at a step S118,whereon a corresponding amount signal T_(p) is outputted at a step S117.

According to the teachings of the present invention described above,there can be realized a fit of sufficient degree between themaneuverability demanded by the driver and the actual maneuvering of thevehicle which follows the demand.

We claim:
 1. An apparatus for controlling driving power of a vehiclehaving an engine, a transmission, means for detecting vehicle speed andmeans for detecting the stroke of an accelerator pedal, comprising:firstmeans for obtaining a desired driving torque of a vehicle required by adriver on the basis of a detected value of vehicle speed and a detectedvalue of accelerator pedal stroke; and second means responsive to saidfirst means for controlling the transmission gear ratio of saidtransmission and engine torque so as to drive an engine substantially ina smallest fuel-cost ratio region on the basis of the obtained desireddriving torque.
 2. An apparatus according to claim 1, wherein said firstmeans includes a map defining desired driving torques for variousvehicle speeds and accelerator pedal strokes, and means for retrieving avalue of driving torque from said map on the basis of a detected valueof vehicle speed and a detected value of accelerator pedal stroke toobtain a desired driving torque.
 3. An apparatus according to claim 1,wherein said second means includes a map defining transmission gearratios of the transmission for various vehicle speeds and desireddriving torques, means for retrieving a value of transmission gear ratiofrom said map on the basis of a detected value of the vehicle speed andthe obtained desired driving torque to obtain a transmission gear ratio,and means for determining an engine torque to be controlled on the basisof the obtained transmission gear ratio and desired driving torque. 4.An apparatus according to claim 1, wherein said second means includes afirst map defining transmission gear ratios of the transmission forvarious vehicle speeds and desired driving torques, a second map fordefining relations among engine rotation speeds, engine torques andfuel-cost ratios, means for retrieving a value of transmission gearratio from said first map on the basis of a detected value of thevehicle speed and the obtained desired driving torque to obtain atransmission gear ratio with reference to said second map, and means fordetermining an engine torque to be controlled on the basis of theobtained transmission gear ratio and desired driving torque.
 5. Anapparatus according to claim 1, further comprising means for controllinga flow rate of air sucked into the engine in the case of changing thetransmission gear ratio so as to maintain the desired driving torque. 6.An apparatus according to claim 1, wherein said second means controlsthe transmission gear ratio and the engine torque so as to provide ahigh engine torque or a low engine negative pressure.
 7. An apparatusaccording to claim 1, further comprising selecting means, responsive toan accelerator pedal stroke of zero, for selecting a transmission gearratio in accordance with a brake pedal stroke.
 8. An apparatus accordingto claim 7, wherein said selecting means includes means for controllingthe engine speed so as to maintain the vehicle speed when thetransmission gear ratio is changed.
 9. An apparatus according to claim7, wherein said selecting means includes means for setting thetransmission to a neutral position when both the acceleration pedalstroke and the brake pedal stroke are zero.
 10. An apparatus accordingto claim 1, wherein the transmission is an automatic transmission. 11.An apparatus according to claim 1, wherein the transmission is astepless automatic transmission.
 12. An apparatus according to claim 1,wherein the transmission is a stepwise automatic transmission.
 13. Anapparatus according to claim 1, further comprising means for controllinga transmission gear ratio so as to maintain the desired driving torquewhen an air-fuel-ratio of a mixture in the engine is changed.
 14. Anapparatus according to claim 13, wherein said means for controlling atransmission gear ratio includes means for lowering a transmission gearratio when an air-fuel-ratio of the mixture falls into a lean state. 15.A method for controlling the driving power of a vehicle having atransmission coupled to an engine, comprising the steps of:obtaining adesired driving torque of a vehicle required by a driver on the basis ofa detected value of vehicle speed and a detected value of acceleratorpedal stroke; and controlling a transmission gear ratio of thetransmission and engine torque so as to drive the engine substantiallywithin a smallest fuel-cost ratio region on the basis of the obtaineddesired driving torque.
 16. A method according to claim 15, wherein saidstep of obtaining includes a step of providing a map defining desireddriving torques for various vehicle speeds and accelerator pedalstrokes, and a step for retrieving a value of driving torque from saidmap on the basis of a detected value of the vehicle speed and a detectedvalue of the accelerator pedal stroke to obtain the desired drivingtorque.
 17. A method according to claim 15, wherein said step ofcontrolling includes a step of providing a map defining transmissiongear ratios of the transmission for various vehicle speeds and desireddriving torques, a step of retrieving a value of transmission gear ratiofrom said map on the basis of a detected value of the vehicle speed andthe obtained desired driving torque to obtain a transmission gear ratio,and a step of determining an engine torque to be controlled on the basisof the obtained transmission gear ratio and desired driving torque. 18.A method according to claim 15, wherein said step of controllingincludes a step of providing a first map defining transmission gearratios of the transmission for various vehicle speeds and desireddriving torques, a step of providing a second map for defining relationsamong engine rotation speeds, engine torques and fuel-cost ratios, astep of retrieving a value of driving torque from said first map on thebasis of a detected value of the vehicle speed and the obtained desireddriving torque to obtain a transmission gear ratio with reference tosaid second map, and a step of determining an engine torque to becontrolled on the basis of the obtained transmission gear ratio anddesired driving torque.
 19. A method according to claim 15, furthercomprising a step of controlling a flow rate of air sucked into theengine in the case of changing the transmission gear ratio so as tomaintain the desired driving torque.
 20. A method according to claim 15,wherein said step of controlling controls the transmission gear ratioand the engine torque so as to provide a high engine torque or a lowengine negative pressure.
 21. A method according to claim 15, furthercomprising, when the accelerator pedal stroke is zero, a step ofselecting a transmission gear ratio in accordance with a brake pedalstroke.
 22. A method according to claim 21, wherein said step ofselecting includes a step of controlling the engine speed so as tomaintain the vehicle speed when the transmission gear ratio is changed.23. A method according to claim 21, wherein said step of selectingincludes a step of setting the transmission to a neutral position whenboth the acceleration pedal stroke and the brake pedal stroke are zero.24. A method according to claim 15, wherein the transmission is anautomatic transmission.
 25. A method according to claim 15, wherein thetransmission is a stepless automatic transmission.
 26. A methodaccording to claim 15, wherein the transmission is a stepwise automatictransmission.
 27. A method according to claim 15, further comprising astep of controlling the transmission gear ratio of the transmission soas to maintain a desired driving torque when an air-fuel-ratio of amixture in the engine is changed.
 28. A method according to claim 15,wherein said step of controlling the transmission gear ratio includes astep of lowering the transmission gear ratio when an air-fuel-ratio ofthe mixture falls within a lean state.
 29. An apparatus for controllingdriving power of a vehicle having an engine and a transmission,comprising:means for detecting vehicle speed and the stroke of anaccelerator pedal; and means for controlling the transmission gear ratioof said transmission and engine torque so as to drive an enginesubstantially in a smallest fuel-cost ratio region on the basis of adetected value of vehicle seed and a detected value of accelerator pedalstroke.
 30. An apparatus according to claim 29, wherein said means forcontrolling includes a first map defining transmission gear ratios ofthe transmission for various vehicle speeds and values of acceleratorpedal, means for retrieving a value of transmission gear ratio from saidfirst map on the basis of a detected value of the vehicle speed and adetected value of the accelerator pedal to obtain a transmission gearratio, and means for determining an engine torque to be controlled onthe basis of the obtained transmission gear ratio and the detected valueof the accelerator pedal.
 31. An apparatus according to claim 29,wherein said means for controlling includes a first map definingtransmission gear ratios of the transmission for various vehicle speedsand values of accelerator pedal, a second map for defining relationsamong engine rotation speeds, engine torques and fuel-cost ratios, meansfor retrieving a value of transmission gear ratio from said first map onthe basis of a detected value of the vehicle speed and a detected valueof the accelerator pedal to obtain a transmission gear ratio withreference to said second map, and means for determining an engine torqueto be controlled on the basis of the obtained transmission gear ratioand the detected value of the accelerator pedal.
 32. A method forcontrolling the driving power of a vehicle having a transmission coupledto an engine, comprising:means for obtaining a desired driving torque ofa vehicle required by a driver on the basis of a detected value ofvehicle speed and a detected value of accelerator pedal stroke; andmeans for controlling a transmission gear ratio of the transmission andengine torque so as to drive the engine substantially within a smallestfuel-cost ratio region on the basis of the obtained desired drivingtorque.
 33. A method according to claim 32, wherein said means forobtaining includes means for providing a map defining desired drivingtorques for various vehicle speeds and accelerator pedal strokes, andmeans for retrieving a value of driving torque from said map on thebasis of a detected value of the vehicle speed and a detected value ofthe accelerator pedal stroke to obtain the desired driving torque.
 34. Amethod according to claim 32, wherein said means for controllingincludes means for providing a map defining transmission gear ratios ofthe transmission for various vehicle speeds and desired driving torques,means for retrieving a value of transmission gear ratio from said map onthe basis of a detected value of the vehicle speed and the obtaineddesired driving torque to obtain a transmission gear ratio, and meansfor determining an engine torque to be controlled on the basis of theobtained transmission gear ratio and desired driving torque.
 35. Amethod according to claim 32, wherein said means for controllingincludes means for providing a first map defining transmission gearratios of the transmission for various vehicle speeds and desireddriving torques, means for providing a second map for defining relationsamong engine rotation speeds, engine torques and fuel-cost ratios, meansfor retrieving a value of driving torque from said first map on thebasis of a detected value of the vehicle speed and the obtained desireddriving torque to obtain a transmission gear ratio with reference tosaid second map, and a step of determining an engine torque to becontrolled on the basis of the obtained transmission gear ratio anddesired driving torque.